Intermolecular Forces Strike Again

When it came to pouring the asphalt fuel grains, I did not anticipate how long it would take.  Starting with approximately ten pounds in two large chunks, I pulled/sliced the bitumen (asphalt without the gravel-just the tar) in small pieces.  This was more difficult than I thought... the long, high mass hydrocarbons in the asphalt would not let go of each other.  With a teflon pan I started to melt the chunks of asphalt over a hot plate, trying to evenly heat the black pitch into a uniform liquid.  This proved daunting, as the asphalt much preferred the solid state, and refused to melt.  After probably half an hour of coaxing, I filled half of one phenolic tube with liquid asphalt, leaving another tube and a half to go.  It became apparent that it would just be easier to pick up the asphalt on an aluminum stick and melt it with a blowtorch than to use a pan.  The whole time, between the spatters of hot pitch (the same material they used to tar and feather people in the days of old; I can only imagine the horrific burns they suffered across their bodies in addition to the public shaming), I was continuously taunted by the pesky properties of this hydrocarbon amalgamation. Next time I'll just drive to the La Brea tar pits and dip the grains in-it'd almost be faster that way.

Melting Asphalt

Another major objective was to drill/tap a hole in the bulkhead where a pressure transducer could fit.  This was necessary because without the pressure transducer, I would not be able to quantify force/pressure vs time in each rocket testing.  The end result was not pretty, but works.  However, I made a terrible mistake and partially destroyed the threads that allow the bulkhead to screw into the combustion chamber.  The damage was mostly repairable and I avoided having to start all over in making/modifying a new bulkhead.

Bulkhead with Transducer Fitting

Later in the week, I began to experiment with the HTPB fuel grains.  HTPB (hydroxyl-terminated polybutadiene) is a liquid rubber base, and in the presence of MDI (Methylene Diphenyl Diisocyanate), cures rapidly into a solid rubber.  I used other additives in making the fuel grains.  I planned to make two HTPB basic grains, and one with powdered magnesium as a fuel additive.  Each fuel grain followed a basic mixture as follows:

40g HTPB
8g MDI
2g CuO (cupric oxide), a stabilizer and fuel supplement
~20mg Triphenyl Bismuth, a contaminant that allows for quicker ignition

The Mg+HTPB grain also included 5g Mg.  This was all relatively easy, with the exception of extracting MDI from its storage container.  The barrel is very tightly sealed, as MDI reacts very quickly with water, tearing apart the isocyanate and rendering the MDI useless.  Additionally, MDI should be kept in a controlled climate, meaning the barrel is housed in a foam igloo.  MDI also has very low viscosity, making extraction via vacuum somewhat difficult, as the vacuum seal was not perfect and allowed MDI to drip very quickly back into the barrel and out of our reach.  Oh, and it's harmful to skin and lungs, pretty harshly so.  Anyways, after mixing all the components, I cast the rubber into the phenolic tubes, each capped with aluminum tape to prevent leakage.  They look like huge shotgun shells.

The Fuel Grains Curing (well, not really curing....)

After this, Robert and I constructed the control box, a task that we did incorrectly.  I have to go move some wires and resolder after I post this.  Simple fix.

Fast forward to today, after a long weekend.  The nitrous valve is mounted, the tubing is in, the bottle is oxygen-cleaned to ensure safety.  All this would be great, but there was one large complication. 

The HTPB grains didn't cure.... Critical Mission Failure.

Left: Failed Fuel after Extraction. Right: Empty Phenolic

The two pure rubber mixes were still incredibly liquid, which by a strange twist of fate, was much better than the magnesium-HTPB grain.  How do I describe it?  The Mg grain was sticky like glue, stretchy like putty, and is like bread dough from hell. And toxic.  But oh it's a very nice color... I think I ought to sell it as a premium automobile paint color... Anyways... since I didn't want to waste the phenolic tubing by throwing away the whole grain, I had to remove the fuel.  What a task that was.  After half an hour, it was all out.  The intermolecular forces really taunted me there.  I can imagine all those electrons, each participating in a subatomic tug-of-war against me, completely determined to make me waste my time. 

Tomorrow I will revisit HTPB.  I am increasing the proportion of MDI to be 20% by weight, hopefully enough to fully cure each grain.  I want to be a chemical engineer so I better get used to finicky reagents. 

I am on schedule to fire by Thursday, or Monday at the latest.  Thanks for viewing.